Lithium ion battery cathode ceramic materials have been an intriguing research field for many years. Out of various cathode materials, lithium transition metal oxides represent the most successful category of cathode materials. The crystal structures of lithium transition metal oxides can be a layered structure with a chemical formula of LiMO2 (where M is, for example, Mn, Co, and/or Ni) or a three dimensional spinel structure with a typical chemical formula of LiM2O4 (M is, for example, Mn). Both layered structure and spinel structures include a framework of the transition metal and oxygen, into which the lithium ions are intercalated.
Lithium ion battery cathode ceramic materials, for example, lithium cobalt oxide, lithium nickel oxide or lithium cobalt nickel oxide, have excellent basic performance for energy storage. However, these materials also have drawbacks, such as insufficient safety, in terms of thermal stability and overcharge performance. To solve these problems, various safety methods have been introduced, which include shut-down functions of separators, additives to the electrolyte, safety protection circuits and PTC (Positive Temperature Coefficient) devices. Unfortunately, all these methods were designed to be used under conditions in which the charging capability of the cathode active material is not too high. Thus, when the charge capability of the cathode active material is increased to meet the increasing demand for high capacity in such batteries, it can cause deterioration in the safety of these systems.
On the other hand, the operation of an electrochemical battery always generates an interface layer between the active cathode material and the electrolyte, called a solid electrolyte interface (SEI). High voltage operation can easily destroy this interface layer, leading to poor cycling performance and capacity loss. Thus, controlling and stabilizing SEI formation and structure is still of great importance and practical interest.
Furthermore, some active manganese-containing cathode materials, like lithium manganese oxide, when directly in contact with the electrolyte, have a problem of manganese dissolution into the cell electrolyte solution during cell operation. This may cause capacity fading, i.e., loss of capacity through repeated charging and discharging cycles.
To overcome the above drawbacks, a core/shell structure has been suggested to improve the cycle life and safety of lithium batteries. The formation of a passivation shell on active cathode ceramic particle surface (the core) can provide structural and thermal stability in highly delithiated (discharged) states, thus the cycle life and safety may be improved. There are various shells that have been described for cathode ceramic particle surfaces, which include shells formed of, for example, barium titanate (BaTiO3), lithium iron phosphate oxide, and gradient LiCoO2. Most of these shell formation schemes either use expensive raw materials or employ a complicated process, or both. In addition to the active material shells described above, inert metal oxide shells have been investigated over a long time. The inert metal oxide shell formation is a relatively cheaper process. Various inert metal oxide shells, such as TiO2, Al2O3, MgO and ZnO, have been prepared on ceramic particle surface through so-called heterogeneous nucleation wet chemistry. However, current heterogeneous nucleation to form inert oxide shells is not controllable, particularly in that known processes do not provide any way to control the shell thickness with acceptable precision. The inert metal oxide shells by definition are not electrochemically active—meaning they do not facilitate ion or electron transport. At the same time, such shells should not interfere with the operation. If a too thick and/or too dense inert shell is formed, the resistance of the shell can limit the charging and discharging rate capability of the electrode and cell performance would deteriorate. Current processes for deposition of aluminum oxide (and other inert oxide) by heterogeneous nucleation using aluminum nitrate (or other aluminum salts), involves ion-exchange between Li cations in the active ceramic material and Al ions in the process solution. This may cause Li ion loss from the active ceramic material, waste generation and possible generation of cathode structure defects when the shell is deposited.
Thus, the problem of how to provide an oxide coating on an inorganic substrate, such as the active ceramic material in a lithium ion battery cathode, has been long-standing and not satisfactorily solved to date.